1. Field of the Invention
The invention relates to the field of earth boring tools and in particular to drag bits in which a plurality of cutters are sequentially exposed and used to cut the rock formation.
2. Description of the Prior Art
The lifetime and ability of a rotating drag bit used in mining or petroleum applications is invariably limited by the durability of the type of rock cutting element that can be brought to bear for cutting the rock. The bit wears out and ceases to cut because the cutting elements wear out or lose their cutting edges. The bit and the drill string must then be tripped to the well's surface and a new bit installed, followed by a return trip of the drill string downhole. The periodic need to replace worn drill bits significantly adds to the cost of drilling operations.
Furthermore, there is presently no universal cutting element which is capable of cutting every kind of rock formation which can be encountered. Various styles of cutting elements and drag bits are optimized to cut various types of rock formations. Therefore, one bit may be efficient in soft gummy formations, but is of very little utility in hard abrasive formations. Other designs of bits would cut well within hard abrasive formations, yet ball up and fail to cut efficiently in soft formations. In still other applications the formation is stratified so that layers of hard and soft rock are alternated. Generally, when a radically different type of rock formation is encountered downhole and the bit ceases to effectively cut, it still must be retrieved to the well's surface and exchanged for a bit suitable for the rock formation which is encountered even though the previously installed bit is not worn out. This substitution also significantly contributes to the cost of drilling operations.
In an attempt to solve these problems, the prior art has devised a number of different bit designs. For example, Cortinas, "Drill," U.S. Pat. No. 1,029,491 (1912) shows a bit in FIG. 5 of that patent having a plurality of drag cutters or blades 17 on its lower end. Wedge-shaped stops 26 are disposed against the unexposed blades to lock the blades in position. Stops 26 in turn are each coupled to a piston 23 and 24. The drill is operated like a conventional drag bit until the lowermost exposed blades are worn away. The drill string and drill are then pulled upwardly within the bore hole and allowed to drop sharply against the end of the rock formation. After being dropped, the drill is rotated to cause bits 16 and 18 to rotate and lift wedge-shaped stops 25 and 26. The result of the rotation will present a sharp blade in each bit position at which point stops 25 and 26 will then be urged by springs 31 into a locked position to prevent reverse rotation during the normal drilling operations.
While showing a means for rotating a new set of cutting elements into operative position, Cortinas illustrates a bit design which relies upon a jarring impulse for operation and is therefore unreliable and furthermore has a limited torque load carrying capability.
Coalson, "Drill Bit," U.S. Pat. No. 3,847,236 (1974) shows a double ended drill bit with two sets of roller cones. After one drill bit becomes dull, a carriage is rotated to expose an upper bit to the bottom of the bore hole. A carriage is rotated to expose the upper bit to the bottom of the bore hole. The carriage is rotated by raising the bit housing in the bore hole in order to provide a space above the bottom of the bored hole to allow rotation of the carriage. Fluid pressure is then increased within the tool and acts upon a hydraulic piston to place the tool in a configuration where the carriage can be rotated. A spring motor is then provided for rotating the carriage to orient the new roller cone bit toward the rock formation.
Coalson, however, fails to show any means for presenting new cutting elements on different segments of the same cone. Furthermore, Coalson is a roller cone bit which cuts by a crushing mechanism as opposed to the shearing mechanism employed in drag bits.
Hildebrandt, "Combination Drill Bit," U.S. Pat. No. 3,066,749 (1962) describes an extensible cutter blade mounted within the body of the bit. The extensible blades are used when cutting through soft formations and are automatically advanced with respect to the body of the bit to continuously present a fresh cutting surface to compensate for wear. Otherwise cutting action is effectuated through a pair of conventional roller cones. The extensible blade can be selectively brought into operation or retracted therefrom.
However, in Hildebrandt the mechanism for extending the blade in soft formations is independent of the roller cone cutters and presents an unbalanced cutting face on the bit.
Evans, "Drill Bit With Yielding Support And Force Applying Structure For Abrasion Cutting Elements," U.S. Pat. No. 4,386,669 (1983) illustrates the prior art use of different types of cutting elements on the same bit. However, Evans does not show the use of such cutters in contact with the rock formation as the function of the need of the user or of the hardness of the formation which is being cut. In FIG. 3, for example, of Evans, Stratapax cutters 94 are used as drag cutters in combinations with a compression cutter 76. The FIG. 4 of Evans, abrasion cutters 94 are used to cut the gauge while compression cutters 76 on roller cones are provided for primary cutting.
However, Evans fails to show any type of mechanism whereby one type of cutter can be selectively withdrawn and replaced by another.
Demo, "Rotary Shock Wave Drill Bit," U.S. Pat. No. 3,250,337 (1966) shows a bit wherein cutting members 24 are rotated at approximately twice the angular velocity of bit 11. Rotation of cutting member 24 is synchronized by an interlocking timer disk 53. Although Demo shows a drill bit with roller cones that are used in a manner, at least in part, to cut through shearing, there is only a single type of cutter on Demo, and no means for selectively bringing a distinguishable type of cutter into play and maintaining it in exclusive cutting engagement with the rock formation.
What is needed is a simple and rugged mechanism capable of withstanding the torque loads commonly encountered in contemporary drilling operations, and which includes a means for selectively bringing into play distinguishable sets of cutters so that the lifetime of the bit is extended, or so that the number of type of rock formation which can be efficiently cut is increased.